The present invention relates to a process for the preparation of allophanate, and to an allophanate and a polyisocyanate composition of lowered viscosity comprising the allophanate which is intended for two-component coating compositions, in particular paint compositions.
The invention relates in particular to a process for the preparation of allophanate by reaction of one (or more) isocyanate(s) with at least one monoalcohol comprising an ether or polyether functional group, the reaction being catalyzed by a bismuth-comprising compound.
Polyisocyanates are widely used in the coating industry, in particular in the paint industry, due to their numerous properties. The use is known in particular, as curing agents, of polyisocyanates comprising isocyanurate groups, due to their crosslinking capability.
However, the compositions of this type, obtained by trimerization of an isocyanate, have a relatively high viscosity which necessitates the use of a substantial amount of solvent.
In point of fact, new regulations relating to monitoring of the environment require the reduction of volatile organic compounds.
One of the possibilities in meeting these requirements consists in limiting the degree of conversion of the starting isocyanates, in particular of the diisocyanates, in order to minimize the formation of heavy compounds (polycondensates with a higher degree of polymerization, more particularly comprising more than one trimer unit) present in the trimerization medium which are responsible for the increase in the viscosity. To this end, the amount of catalyst is reduced for a set reaction time or the reaction time is reduced for a given amount of catalyst, in order to increase the true cyclotrimers/heavy compounds ratio.
The applicant company already markets products of this type, HDT (Hexamethylene Diisocyanate Trim er) and HDB (Hexamethylene Diisocyanate Biuret), denoted under the acronym LV for “Low Viscosity”.
The disadvantages of these procedures are, firstly, a significant fall in the productive output and, secondly, an increase in the cost due to the amount of catalyst used for a given amount of isocyanurates.
The proposal has also been made, in the European patent applications EP 0 524 500 and EP 0 524 501, to carry out an allophanation reaction on a trimerization mixture or to carry out the trimerization in the presence of alcohols, which results in polyisocyanate mixtures comprising isocyanurate functional groups claimed as exhibiting a low viscosity. This process results in the formation of allophanates of polyfunctional isocyanates, which compounds increase, in a significant and harmful fashion, the viscosity of the compositions.
It is also known to add allophanates to compositions formed of polyisocyanates comprising isocyanurate groups, in order to reduce their viscosity. Thus, on carrying out the catalytic (cyclo)trimerization reaction and the allophanation reaction separately, the formation of allophanates of polyfunctional isocyanates, which compounds increase, in a significant and harmful fashion, the viscosity of the composition, is avoided.
Allophanates are generally obtained by reaction of a compound comprising an alcohol functional group with an isocyanate, followed by reaction of the carbamate functional group thus obtained with a new isocyanate molecule.
Processes for obtaining allophanates are known, which processes generally employ catalysts.
For example, it is known to prepare allophanates by using a quaternary ammonium as catalyst. Thus, allophanates are formed but trimers are also formed, which is undesirable.
It is also known to prepare allophanates by using tin-based compounds as catalysts. The reaction requires many hours and the tin-based compounds are toxic.